attr('defaultValue') is returning undefined using jQuery 1.6.3

I have a simple script in jQuery that works perfectly with jQuery 1.5.2 as you can see in this jsFiddle. What is supposed to happen is that when you bring focus to the text field, the default value is removed. And when if you leave the field blank, the original default value is put back in place.

However, the same exact code, where only jQuery 1.6.3 is used instead, is not working. (Not working means that the default value remains in the text box until you manually delete it as you can see in this jsFiddle.

Attributes exist in the HTML source code.
Properties exist in the DOM tree.

When the browser parses the HTML source code, the HTML <input> element is interpreted and a corresponding HTMLInputElement DOM node is created. This DOM element contains dozens of properties ('defaultValue' being one of them).